Arctic melting: the science behind the ice

The National Snow and Ice Data Center (NSIDC) ended the wait on Wednesday, announcing what many already had expected: that the Arctic sea ice extent had not only hit a record low, it had dramatically receded to levels not previously anticipated. But what does this mean?

To help understand the science behind the record melt, the Greenpeace ship Arctic Sunrise took NSIDC scientist Dr Julienne Stroeve and Nick Toberg, an ice scientist at Cambridge University, into the Arctic to facilitate further research. We recently caught up with them to discuss the changes happening at the top of the planet.

Why is this year so different, has it been particularly warm, like in 2007 when the previous record was set?

Julienne: This summer the weather was not particularly warm or conducive to ice loss, yet 2012 shattered the 2007 minimum. This tells me that the ice was likely very thin and vulnerable to melting out even under more normal weather conditions. As the Arctic loses more of its store of old, thick ice, it is being replaced with thinner first year ice, which is more prone to melting out each summer. While natural climate variability makes it difficult to predict if a new record low will be set again next summer, it is becoming increasingly clear that the Arctic is in a new climate state with more seasonal ice loss each summer.

Nick: In 2007 the weather conditions were right for the ice to shrink. Last year there were no unusual weather conditions, but the ice came close to beating the 2007 minimum. We are now seeing ice that is 'pre-conditioned' to dip lower than the previous year's extent. This means that the ice pack is in such a weakened state, composed of mostly young thin ice, that it cannot recover by re-thickening enough over the winter to survive the summer melt season. The long-term trend is accelerated sea ice retreat, resulting in ice-free summers in our lifetimes.

What are the different functions of ice thickness (volume), and ice extent (surface area)?

Julienne: Ice extent is important for reflecting the sun’s rays and keeping the atmosphere cool. But if we lose ice volume, the ice becomes very thin and melts more easily. The decrease in thickness is why we are losing so much surface area of the ice.

As surface area continues to shrink how is melting affected?

Julienne: The heat that was previously reflected by the ice is now mostly absorbed by the ocean, but in autumn when it starts to freeze again, the heat gained by the ocean is released back to the atmosphere, leading to warmer air temperatures, more clouds and greater moisture in the atmosphere. More clouds in autumn and winter actually keep the atmosphere even warmer, reducing ice growth.

There were intervals of the Arctic being ice- free 100,000 to 120,000 years ago. Why was that?

Julienne: We don't know if it was entirely ice free then, we know that parts of it may have had less ice than today. We do know that different factors were responsible – more energy from the sun, which is not the case now.

What is the long-term picture?

Nick: If the remaining 4 million sq. km of sea ice disappear in summer, that would equate to adding 20 years worth of CO2 to the atmosphere (at today’s levels of CO2 emissions). In the 1970s, we had 8 million sq. km of summer sea ice, so it has halved in the last 30 years.

Julienne: If we keep warming the atmosphere, the Arctic could be ice free in the summer by 2030. While natural climate variability could increase ice levels temporarily for a few years, it is on a downward long-term trend. This year is significant – we’re on the extreme end of current climate models. Our observation is that sea ice decline is happening much faster than most of the new IPCC models.

What are the impacts on areas of the world beyond the Arctic?

Nick: The ice is a big cooling system and the earth’s weather patterns depend on it. There will not be the same dispersion of cold and heat – for example, warm water currents will not move past Britain any more – making it less temperate. Britain is actually at the same latitude as Siberia and is made warmer by the North Atlantic current.

Julienne: The Arctic drives Northern Hemisphere weather patterns. Open water absorbs the sun's energy, which is transferred back to the atmosphere in autumn, causing the Arctic to warm further. Arctic warming changes the difference in temperature between the equator and the pole, large-scale air circulation patterns such as the jet stream. These changes can result in more persistent weather conditions, such as droughts, heat waves and flooding.

Another impact of increased moisture in the Arctic atmosphere is the potential for increased precipitation and storms. Years with low summer ice extent tend to be followed by more frequent and more intense storms in the North Atlantic, bringing more precipitation to parts of Europe and Siberia.

How do you think the world will respond?

Nick: It’s disturbing that we have changed the face of the planet in a way we can visually see. But people won’t really change until it affects them on a day to day level.

Julienne: I hope that this year’s sea ice loss will have an impact on the US government and change thinking.